Broken ribbon of lights at bottom of screen in landscape mode

Yesterday I noticed that, when reading in landscape mode on my iPad, there was a broken horizontal ribbon of lights at the bottom of the screen. If I read in portrait mode, the ribbon appears vertically at the left of the screen. What is causing this and can it be fixed? I was going to include a screen shot but don’t see how to attach it. Copying and pasting the photo didn’t work. Thank you.